It is known that certain tricyclic quinoline and naphthyridine derivatives possess antibacterial activities. For example, U.S. Pat. No. 4,767,762, incorporated herein by reference, discloses certain isothiazolo-quinoline derivatives which possess antibacterial activity. However, these and other related novel derivatives have not been known heretofore to be antineoplastic agents.
DNA topoisomerase I and DNA topoisomerase II are enzymes, located in the nuclei of cells, which bind to DNA and alter the configuration or topology of DNA. These enzymes play a key role in the replication, recombination and transcription of DNA necessary for cell growth and reproduction. Topoisomerases also play critical roles in maintaining chromosome and nuclear structure. Recently, several clinically useful anti-tumor agents were found to form cleavable complexes with DNA topoisomerases and DNA in tumor cells and to induce considerable DNA breakage. This damage to the DNA initiates a sequence of events that leads ultimately to the death of the tumor cell. Examples of clinically important antitumor drugs which have been found to affect the breakage-rejoining reaction of mammalian DNA mediated by DNA topoisomerases are anthracenediones such as mitoxantrone, and epipodophyllotoxins such as etoposide and teniposide. It was unexpectedly found that the compounds of the present invention, which are unrelated to known antitumor agents, can induce DNA breakage mediated by topoisomerase II and have cytotoxic activity.